Automotive aluminium casting alloys

Cast aluminium in automotive industry

Casting aluminum alloys are widely used in automotive industry. Aluminum casting is a variation of this that uses only aluminum and aluminum alloys as the liquid metal that is poured into the mold. Aluminum castings are used to make complex and detailed parts very efficiently. Casting aluminum alloys are quite widespread and find more and more applications in modern industry. According to different estimates, up to 20–30% of all aluminum products manufactured worldwide are used for aluminum die castings.

Aluminum casting methods

Among aluminum casting methods sand mold casting, precision casting and high pressure die casting methods are most common used aluminum casting methods (Figure 1).

Figure 1 – The aluminium production methods in automotive industry [1]

Sand casting

Sand casting method is one of the most preferred production methods when not only the automotive sector is considered. There are many parameters that are important in the sand mold casting method. These can be listed as follows; molding process, pattern, sand quality, cores. The molding process is a very important parameter in the sand mold casting method. The process using molding sand creates the compacted sand model cavity. Thanks to the runner system, it is aimed to give the desired geometry by pouring the melted metal alloy into the mold cavity.

High pressure die casting

High pressure aluminum die casting is a manufacturing process in which molten aluminum is injected with a die casting machine under force using considerable pressure into a steel mold or die to form products. In addition, short cycle times are feature advantage of high pressure casting in the production of thin-walled parts with complex and difficult geometric structures. High pressure die casting production is rather fast relative to other aluminum casting processes. Aluminum die casting applications also allow to offer aggressive casting delivery times.

Low pressure aluminum die casting

Low pressure aluminum die casting can be also preferred for the components without concern for high quality. Since the low pressure casting method is longer in terms of cycle time, it is generally preferred for parts with lower annual numbers as well as the production volume. Low pressure casting is the preferred method of manufacturing components such as engine blocks, wheels and  suspension parts.

Aluminium casting alloys

Aluminum casting alloys make it possible to provide a wide range of mechanical properties of products by choosing the chemical composition, as well as, if necessary, heat treatment and strict technological control.

Figure 2 – Typical hypoeutectic AlSi7Mg (AA A356) Microstructure:
Al dendrites surrounded by Al+Si eutectic [2]

Aluminium casting alloys comprise four main families of alloys, which are based on the following alloying systems:

• Al-Si(-Cu)
• Al-Cu
• Al-Mg(-Si)
• Al-Zn(-Si).

Most cast aluminum parts and products are made from aluminum-silicon alloys due to their exceptional casting properties (Figure 2).

Some alloys in the Al-Cu, Al-Mg and Al-Zn series have higher mechanical properties than Al-Si alloys, but their casting characteristics are usually quite poor and very difficult to cast. In particular, they are very prone to hot cracking.

Al-Mg casting alloys

• These alloys have high corrosion resistance. They can be polished to a mirror finish and also anodized.
• They provide high mechanical properties without the need for heat treatment.
• These alloys are usually used in the manufacture of parts using high pressure casting.

Al-Zn-Mg casting alloys

These alloys are used when it is necessary to avoid warping during quenching, since they are thermally hardened by aging at room temperature without prior quenching.

Casting alloys Al-Mg-Si and Al-Zn-Si-Mg

Silicon additives in these alloys in sufficient quantities provide them with good casting properties.

Al-Cu casting alloys

• These are the strongest cast aluminum alloys. Mainly used in the aerospace industry.
• Important parts of automobile suspensions are also made from these alloys.
• These alloys are very susceptible to hot cracking and are very difficult to cast into complex shapes.
• Used in limited quantities for parts of sports cars and other exotic vehicles.

Al-Si casting alloys

1) The most popular casting alloys around the world (see Figure 2).
2) They exhibit excellent casting properties: high fluidity – they fill molds well, are resistant to hot cracking and, in general, are the most technologically advanced casting alloys.
3) The addition of magnesium provides good mechanical properties after heat treatment, and copper improves machinability and increases strength at elevated temperatures.
4) Al-Si-Mg alloys are used for the manufacture of wheel rims, structural castings and suspension components that require high strength and good toughness.
5) Al-Si-Cu(-Mg) alloys dominate the market for powertrain parts and assemblies, such as aluminum engine blocks, cylinder heads, engine cylinder liners and other cast parts for which high strength at elevated temperatures and wear resistance are more important than viscosity.

 Figure 3 –  Schematic Al-Si phase diagram showing the composition ranges
for the most common foundry alloys [2]

Sources:

1. New Trends in Aluminum Die Casting Alloys for Automotive Applications / T.A. BASER, E. UMAY, V. AKINCI – EPSTEM, 2022
2. European Aluminium Association, 2002